Unraveling the cerebellum’s role in multiple sclerosis

Multiple sclerosis (MS) is a neurodegenerative, inflammatory, and demyelinating disease of the central nervous system affecting approximately 2.8 million people worldwide. Cerebellar dysfunction in MS presents with ataxia, ocular movement disorders, tremor, dysmetria, as well as cognitive deficits. Magnetic resonance imaging has improved our understanding of cerebellar dysfunctions in MS and their relation to motor and cognitive impairment. In this short review, we discuss current literature and recent progress in the field of cerebellar dysfunction in MS.


Cerebellar dysfunction in multiple sclerosis
Multiple sclerosis (MS) is a complex disorder of the central nervous system (CNS) with a typical onset between 20 and 40 years [1].Beyond physical disability, which might manifest through unsteady gait, coordination difficulties, weakness, or sensory dysfunctions, impairment in cognitive functions occurs in more than 50% of all MS patients, regardless of clinical phenotype [2][3][4].Cerebellar involvement has been frequently reported in MS and cerebellar dysfunction can be experienced by MS patients in the form of motor signs (e.g.nystagmus, intention tremor, ataxia, and dysmetria) [3] as well as cognitive dysfunction (e.g.impairment in executive function, visual-spatial performance, and linguistic processing) and affects dysregulation [5].Cognitive and behavioral symptoms after cerebellar damage are defined as 'cerebellar cognitive affective syndrome' [5][6][7][8].

Cerebellar pathology in multiple sclerosis
Pathologically, MS is characterized by focal and diffuse inflammation and degeneration in the CNS, which affect both white and gray matter [9].
In MS patients, demyelination commonly occurs also in the cerebellar white and gray matter [3,[10][11][12].Within the cerebellar cortex, focal and diffuse demyelination is extensive, and this appears to be true also in early disease stages as shown by ultra-high-field magnetic resonance imaging (MRI) [13].In addition, similar to the neocortex, intracortical, leukocortical, and subpial bandlike lesions have been described in the cerebellum [10].Recent imaging studies show that there is an increased activation of glia cells in cerebellar lesions -as measured by 11 C-PBR28, a second-generation translocator protein radiotracer used to image neuroinflammation combined with ultra-high-field magnetic resonance-positron emission tomography (MR-PET) [14], and that leukocortical lesions in the cerebellum are independent predictors of total cerebellar cortical volume and cerebellar dysfunction in MS patients [15].The authors acknowledge the low MRI resolution of the applied double-inversion recovery sequence thatcombined with the thinness of the cerebellar cortexlimits the consistent identification of leukocortical lesions [15,16].
As to focal white matter (WM) lesions, they are typically found in the cerebellar peduncles or in the regions of the dentate and olivary nuclei [17,18].Cerebellar WM lesions are common in early phases of MS [15,19] and have been linked to progression from clinically isolated syndrome (CIS) to definite MS [20].Additionally, WM lesions show increased glia activation in MR-PET with PBR28 [14] to a similar extent of what is shown in the forebrain [21].

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Both focal and diffuse demyelination lead to cerebellar volume loss.Some studies report a reduction in the total cerebellar volume in MS patients compared with healthy controls, with a more profound volume loss in progressive MS patients [22][23][24].Volume loss in the posterior part of the cerebellum is mostly associated with cognitive impairment, while motor disturbances are attributed to atrophy of the anterior cerebellum [25,26], see below for a detailed description.

Cerebellar abnormalities and their relation to motor functions in multiple sclerosis patients
Neuroimaging and postmortem anatomical and histology studies laid the foundation to study cerebellar structures and the underlying structure-function relationship.Most imaging studies assessing correlates of motor functions applied morphometric measures of cerebellar or lobular volumes or connectivity measures obtained with diffusion tensor imaging (DTI).DTI is a technique that studies the diffusion of water molecules within a tissue and thereby provides insights into the underlying tissue microstructure [27].In fact, DTI provides metrics such as fractional anisotropy (FA), which is a surrogate measure of axonal integrity in WM.Axial diffusivity (AD) and radial diffusivity (RD) are two additional measures provided by DTI: the first has been related to axonal degeneration in areas that are void of crossing fibers, the second refers to changes in myelin integrity [i.e.demyelination or changes in myelin integrity] [28,29].On the other hand, mean diffusivity (MD) is a bulk measure of average diffusion rate across all spatial directions, which is sensitive to changes in tissue microstructure [28,30].
In an original work by Anderson and coworkers, lower FA and increased MD in the middle cerebellar peduncle was found in progressive MS patients compared with relapsing-remitting MS patients and healthy controls [35].In addition, lower cerebellar WM volume contributed to worse performance in the upper-limb test in progressive MS patients [35].In a small investigation of 49 MS patients and 32 healthy controls, Ruggieri et al. (2020) [36] showed a decrease in FA in the cerebellar peduncles in MS patients compared with healthy subjects.Clinical measures of physical disability (Expanded Disability Status Scale [EDSS], timed 25-foot walk [T25FW]) were associated with lobule-VIIIb atrophy after correction for age, gender, intracranial volume, and body-mass index [36].Cerebellar lobular volumes were positively correlated with lower FA in all cerebellar peduncles in patients [36][37][38].Decreased FA in most WM tracts correlated with worse performance in the Nine-hole peg test (9HPT), a measure of upper-limb functions, in MS patients compared with healthy controls [39].Independent of disease duration and cerebellar lesion load, early relapsing-remitting MS patients showed altered structural connectivity along the C4 region (local cerebellar connection: left I-IV to left VI) with magnetization transfer maps being positively related to arm functions and T1 relaxometry being negatively associated with arm functions, as measured by MS Functional Composite [40].

Cerebellar abnormalities and their relation to cognitive functions in multiple sclerosis patients
Moreover, studies assessing cognitive dysfunctions have reported cerebellar abnormalities in MS patients.Weier and coworkers investigated the relation between cerebellar atrophy, clinical cerebellar signs, and cognitive performance in a cohort of MS patients with and without cerebellar signs.Cerebellar signs were present when the cerebellar functional system score was > 0. Patients with cerebellar signs performed significantly worse on the Symbol Digit Modalities Test (SDMT) and Paced Auditory Serial Addition Test (PASAT) and had lower cerebellar volume compared with patients without cerebellar signs [41].Moroso and colleagues explored the association of FA and MD in lobules VI-VIIb with cognitive performance in cognitively impaired and CIS patients.The authors found lower FA, and higher MD, respectively, in the whole cerebellum, but specifically in lobules VIIb and VIIIb in cognitively impaired MS compared with CIS patients and healthy controls.More than 30% of working memory z-score was explained by FA in the left lobule VI and left superior peduncle.SDMT z-scores explained on average almost 40% of FA in the left lobule VII and right lobule VIII [42].Kalron and coworkers studied the relationship between fall status, cerebellar volume, and cognition in MS patients and found a significant association between visual-spatial memory and left cerebellar volume loss in nonfallers T1 relaxometry and generalized fractional anisotropy (GFA) along C3 region were highly correlated with cognitive performance in early relapsing-remitting MS patients, particularly with verbal memory and information processing speed, assessed by selective reminding test (SRT) long-term storage (GFA positively correlated, T1 negatively correlated), SRT consistent long-term retrieval (GFA positively correlated), and SDMT (GFA positively correlated) [40].

Cerebellar abnormalities and their relation to both motor and cognitive functions in multiple sclerosis patients
Resting-state functional connectivity analysis has been widely used to assess the mechanisms underlying cerebellar dysfunction.
Using a seed-based approach, Pasqua and coworkers parcellated the cerebellum according to its function into a sensorimotor (lobules I-V, VIII) and cognitive cerebellum (lobules VI, VII, IX, and X) [46].Compared with healthy controls, MS patients exhibited altered functional connectivity of both the sensorimotor and cognitive cerebellum and those abnormalities correlated negatively with disability (EDSS, 9HPT, and T25FW) and positively with cognitive performance (SDMT and PASAT).Specifically, the authors found an increase of FC for the cognitive cerebellum in thalamic, temporal, and occipital and a decrease of frontal-insular areas.For the sensorimotor cerebellum, they found an increase in FC in thalamic, prefrontal, and parietal areas, and a decrease in FC in insular and basal ganglia areas [

Cerebellar plasticity and repair mechanisms after rehabilitation programs
Cognitive rehabilitation may enhance neuroplasticity in patients with MS.This might be measured using functional magnetic resonance imaging (fMRI) [51].In a randomized control study, Cerasa et al. [52] showed that targeted computer-based training for attention deficits led to an improvement in attention abilities compared with the placebo group that was associated with increased activity in the superior parietal lobe and posterior cerebellar lobule (lobule IV) but only in the experimental group.The finding that cerebellar rehabilitation enhances cognitive performance was also supported by the study group of Sastre-Garriga et al.
[53].After a five-week cognitive rehabilitation program, an increase in fMRI activity in several cerebellar areas was found and patients performed significantly better on the digit span test and on a composite score of neuropsychological outcomes.

Conclusion
The cerebellum is a critical structure in the CNS, which is involved in multiple sensorimotor and cognitive processes.Alterations in specific cerebellar regions and pathways are related to the development of motor and cognitive dysfunction, as well as of disability, in MS patients [25,26].So far, only a limited number of studies has investigated cerebellar structural abnormalities as well as structural/functional connectivity changes and their relation to sensorimotor and cognitive processes.
Studies varied according to the sample size, inclusion criteria (i.e.low EDSS, limited disease duration), study design (mostly cross-sectional), MS phenotypes, and different MRI techniques (i.e.voxel-based, structural/ functional connectivity, and seed-based approach).
One limitation of the abovementioned studies is the constraint in MRI and analysis techniques [54,55], which often renders the analysis of the cerebellum's surface and its high foliation really challenging.
In most studies, SDMT and PASAT were used to measure cognitive impairment.The need for standardized test batteries with yearly follow-ups is great as verbal memory and visual-spatial memory can be already impaired in early MS stages and have been found to correlate with cerebellar activation [56].
Very little is known about the long-term impact of cerebellar abnormalities and sensorimotor and/or cognitive symptoms.Future prospective studies should investigate the longitudinal evolution of cerebellar abnormalities (i.e.atrophy, lesions) and their relationship with cognitive and sensorimotor symptoms, especially considering the influence of cerebellar network plasticity, cerebellum repair mechanisms, and the effect of DMT.
In conclusion, data on neuropsychological and neuroimaging literature strongly support the link between cerebellar dysfunctions co-occurring with cognitive and sensorimotor deficits in MS patients.More work is needed to better delineate cognitive and sensorimotor impairment and cerebellar pathological measures.In a cohort of 144 relapsing-remitting MS patients that had no clinical disability (EDSS ≤1.5) the authors found increased and decreased patterns of both the sensorimotor and cognitive cerebellum that were associated with structural measures.For the first time, the authors found that structural brain damage was negatively correlated with FC of the cerebellum, indifferent whether FC was increased or decreased compared to healthy controls.

Early predictors of disability and cognition in multiple sclerosis patients: a long-term retrospective analysis
In a multi-method approach, Yang and coworkers assessed the role of cerebellar connectivity abnormalities in two pathological conditions and the impact of specific genes on functional connectivity abnormalities.The study group identified genes that accounted for more than 35% of functional connectivity alterations in MS patients.Overall, they found a decrease of FC between cerebellar secondary motor modules in MS patients..J Clin Med 2023, 12:685.In line with other studies, Virgilio et al. found that MS patients with cerebellar lesions performed worse on cognitive tests.Age, education, cerebellar lesion lad and EDSS were associated with poorer cognitive performance after a mean follow-up period of 16 years.DMT had a protective effect on physical disability (i.e.EDSS) and cognition at the time of the last clinical follow-up with cognitive examination.However, due to the heterogeneity of DMTs no additional analyses were performed to assess the effect of DMT and cognition.49.Schoonheim MM, et al.:

The cerebellum and its network: disrupted static and dynamic functional connectivity patterns and cognitive impairment in multiple sclerosis
•Schoonheim et al. found that cerebellar structural damage was most pronounced in secondary-progressive MS patients.Static FC of the default-mode network and fronto-parietal network was decreased in secondary progressive MS patients while the variability of FC (=dynamic) was increased when compared to early relapsing-remitting patients and healthy controls.High dynamic FC corresponds to a high flexibility of networks, while low dynamic FC can be interpreted as a measure of stability.These results support the hypothesis of a network collapse in later stages.